CODEWITHSUNDEEP
go
rausch
rausch

Reputation: 3398

Where does Go get time zone information from?

I ran into an issue with Go code, related to time zones, that would return either 1900-01-01 01:00:00 +0100 CET or 1900-01-01 00:53:28 +0053 LMT, depending on which machine it is being run:

https://play.golang.org/p/K3ceq1n1KI

I was able to rule out the Go version as source of the difference. Where does Go get its time zone information from?

Upvotes: 1

Views: 2787

Answers (1)

Seva
Seva

Reputation: 2498

Actually it depends.

Check time.LoadLocation() source and the comment above it. Particularly, it says this:

// LoadLocation looks in the directory or uncompressed zip file
// named by the ZONEINFO environment variable, if any, then looks in
// known installation locations on Unix systems,
// and finally looks in $GOROOT/lib/time/zoneinfo.zip.

And you should keep in mind that on a Unix/Posix systems (e.g. Linux), proper time zone data files are always available in "known installation locations". But on other systems there no such locations. As a result, on Windows, LoadLocation zone won't find any time zones (unless either ZONEINFO or GOROOT were configured properly) and will just use default instead (which is UTC, if my memory is correct).

However, the LMT issue you described above is a bit different beast. You see, LMT is not exactly a proper time zone. E.g. see here. And I didn't dig out where exactly it comes from, but I suspect it is related to the fact that the time zone was not established yet at the time that you're converting. So, go seems to be calculating LMT of the place instead.

For example, if you just change year from 1900 to 2000 (or even 1905) in your playground example, it will come out with correct time zone (CET).

As to the difference between machines, I'd say it's pretty arguable what is the most natural way of defining time zone rules before they were actually introduced. As a result, I would imagine that in some time zone databases, start time of the first rule in a time zone will be either omitted or adjusted to extend it further to the past. While others will put an LMT offset there instead. (Even though most if not all of them are derived from the same IANA database in one way or another.)

If you want to ensure exact same behavior on different machines, I'd say building your own zoneinfo.zip and setting ZONEINFO variable should help.

Upvotes: 2

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